1. Field of the Invention
This invention relates in general to a photomask arrangement for preventing reticle patterns from peeling caused by electrostatic discharge damage. More particularly, the invention provides a modified photomask which increases the effective surface area of the reticle patterns by forming a plurality of metal lines on a clear scribe line to connect the adjacent metal shielding layers.
2. Description of Related Art
Photolithography is one of the most important techniques utilized in the manufacture of today's semiconductor integrated circuits. Most semiconductor device structures are defined by this process. In fabricating a transistor, for example, both of the thin film patterns and the impurity diffusion regions are defined by a photolithography process. Therefore, the smallest feature line width that can be achieved by the photolithography process is usually used to determine the capability of a semiconductor plant.
FIG. 1 shows a conventional photolithography process. First, a photosensitive material 12, such as a photoresist layer, is coated on the surface of a silicon wafer 10. A parallel light (indicated by the arrows) then passes through a photomask 14 made of glass or quartz into the photosensitive material 12. An exposor (not shown) is used to provide the parallel light. As is known in the art, the minimum resolution depends on the wavelength of the light source. For example, an I-line (with a wavelength of 3650 .ANG.) stepper normally can provide a minimum resolution of about 0.35 .mu.m.
Next, by using a photochemical reaction of the photosensitive material 12, patterns in the photomask 14 are transferred to the surface of the silicon wafer 10. Usually, the patterns in the photomask 14 are provided by forming a chrome reticle thereon (the shaded area in the drawing). Since the parallel light emitted from an exposor is passed through the photomask 14 into the photosensitive material 12, the photochemical reaction selectively occurs in an area of the photosensitive material 12 not shielded by the chrome reticle. After developing, the photosensitive material 12 has the desired patterns corresponding to those of the photomask 14. The patterned photosensitive material 12 is then used as a mask for defining various structures in the silicon wafer 10.
This is the basic principle of a photolithography process that transfers the patterns of the photomask 14 into the photosensitive material 12. Generally, a so-called positive-type photolithography process is used for fabricating a device having a feature size smaller than 3 .mu.m. That is, the remaining pattern of the photosensitive material 12 is the same as that of the photomask 14.
FIG. 2 is a cross-sectional view schematically illustrating a conventional photomask. As shown in the drawing, the photomask includes a glass (or quartz) 20, a chrome film 22, and a pellicle 28 placed on a base 30 which is fixed by a supporting frame 26. The glass 20 is the body of the photomask. The chrome film 22 is formed on the surface of the glass 20 to provide the reticle pattern. The pellicle 28 is used to segregate particles from the surface of the chrome film 22. Thus, any particles adhering to the photomask will not act as a shield during the photolithography process because they are out of focus. Furthermore, a pod 24 covers the glass 20 of the photomask to prevent it from being stained with particles which generally cause repeating defects on the silicon wafer.
During handling of the photomask, such as during inspecting, cleaning and transporting, the operators may rub the pod 24 with their PVC glove by accident. Static charges are formed on the surface of the pod 24 which, in turn, induce corresponding charges on the surface of the glass 20 and the chrome film 22. Because the chrome film 22 has a small area, the induced charges on the glass 20 and the chrome film 22 form a relative high static potential thereon. Generally, a conventional photomask can sustain a static potential no more than 14 kv/inch. However, the static potential caused by the PVC glove would be greater than 15 kv/inch. This high static potential finally causes electrostatic discharge damage to the reticle pattern, i.e., the chrome film 22 peels.
FIG. 3A illustrates, in top view, the chrome film layout of a conventional photomask. As can be seen in the drawing, there are four spaced-apart chrome films 22a, 22b, 22c, and 22d formed on a photomask. A clear scribe line 32 is formed around these four chrome films 22a, 22b, 22c, and 22d for their isolation. Since each of the chrome films 22a, 22b, 22c, and 22d has a small area and no line is formed therebetween, there is no path to spread the static charges caused by an accidental rub from one chrome film to the other. Thus, the static charges form a relative high static field on the chrome films 22a, 22b, 22c, and 22d, causes electrostatic discharge damage.
Referring to FIG. 3B, a break on the chrome film 22 caused by a high static field is depicted. As shown in the drawing, the break usually appears at a corner 34 of the chrome film 22, i.e., the area where the electrostatic discharge occurs.
Accordingly, several modified photomask designs have been disclosed to overcome the problem of electrostatic discharged damage. One of the modified photomasks is shown in FIG. 4. A gold film 40 having a thickness of about 60 .ANG. is sputtered on the surface of the photomask to connect all of the chrome films 22. However, there are some drawbacks to forming such gold film 40 on the surface of photomask. First, it is not easy to form a gold film 40 with a thickness of only 60 .ANG. using today's process techniques. The yield from fabricating this kind of photomask is poor. Furthermore, the additional step of forming a gold film 40 on the surface of a photomask increases the manufacturing complexity and cost.
Another approach to the problem has been to neutralize the static charges on the chrome film 22 with opposite-type ions generated by an ionizer. The static field can be reduced since the amount of static charge on the chrome film 22 is decreased. Hence, the reticle patterns of photomask can be prevented from incurring electrostatic discharge damage.
FIG. 5 is a chart showing the relationship of the static potential and the ionizer treatment time. As shown in the drawing, the static potential of the chrome film 22 can be reduced from 15 kv/inch to less than 1 kv/inch in 20 minutes. However, the chrome film 22 is damaged before the ionizer treatment since the static potential has increased to more than 14 kv/inch. Therefore, a break caused by the high static potential occurs at the corner of the chrome film 22 as well.